Customize Your Query While Importing Data from a Database, Hadoop, or Google BigQuery

While you are building a query for importing data, you can customize the query manually.

Build a query in the Query Builder pane of the Import from Tables dialog.
Click Edit SQL.
Click Yes to exit Build Query mode. The query text appears in the Query Builder pane.
Make changes to the query as necessary.
Write the query as a single-pass SELECT statement. Other standard SQL statements, such as CREATE, are not supported by Google BigQuery. See the BigQuery Query Reference for a list of supported SQL clauses, functions, and operators.
If you are aggregating over a column with a large number of distinct values, BigQuery recommends using the GROUP EACH BY clause instead of a GROUP BY clause.
If you are creating a join between tables, the syntax of the JOIN clause that you write depends on the size of the tables that you are joining. See Combine Data in Tables with Joins in Google BigQuery for more information.
Click Execute SQL to run the query. A preview of your query results appears in the Preview pane.
When you are satisfied with the results, you can finish importing your data.